1. Field of the Invention
The present invention relates to a positioning system for receiving signals from a positioning satellite or an apparatus similar to a positioning satellite to perform the positioning thereof, and in particular, to a positioning system for performing positioning with respect to a moving object. A positioning satellite or a satellite referred to in the following description shall include an apparatus similar to a positioning satellite unless specifically described.
2. Description of the Related Art
In positioning performed by receiving signals from a positioning satellite or from an apparatus similar to the positioning satellite, one of the factors which affects the precision thereof is a satellite arrangement. A degree of influence on the precision of the satellite arrangement is called “DOP: Dilution of Precision, ” which can be calculated using schematic trajectory information of the positioning satellite. In a conventional positioning system, the DOP is generally used as an index for selecting the positioning satellite in order to use it upon positioning. This is indicated, for example, on page 93 of “Revised “Basis of GPS surveying”, Jun Tsuchiya and Hiromichi Tsuji,” Japan Association of Surveyors.
A conventional positioning system will now be described with reference to a drawing. FIG. 10 is a simplified block diagram showing an example of a configuration of an N channel receiver of positioning satellite signals that forms a part of the conventional positioning system.
In FIG. 10, reference numeral 1 denotes an N channel receiver of positioning satellite signals, 81 denotes an antenna, 82 denotes an amplifier, 83 denotes a mixer, 84 denotes an IF, 85 denotes an AD converter, 86 denotes correlation detecting DLLs and 87 denotes decoders.
Operations of the conventional positioning system will be described next with reference to a drawing.
Electric waves from each positioning satellite have a substantially identical frequency. However, the electric waves can be identified by a correlator because they are not CDMA converted by particular data. Since the frequency of electric waves from each satellite is fluctuated by the Doppler effect of the like, it is necessary to follow it by the DLL. Received data of each satellite is thereafter obtained by a decoder.
Since a detected part can be digitized, detection circuits normally in the order of 8 channels to 16 channels can operate simultaneously to follow signals from the individual positioning satellites. Since the number of positioning satellites is larger than the number of channels of a receiver, each channel does not always follow a particular positioning satellite. Therefore, an N channel output of a receiver includes an identification number of a positioning satellite. The received data further includes a pseudo distance ρ between the positioning satellite and the receiver, trajectory parameters of the positioning satellite or the like.
FIG. 11 shows a method of calculating a GDOP (Geometrical DOP) that is a kind of the DOP.
A matrix A shown in FIG. 11 is generally called a design matrix. Each line of the matrix A corresponds to each positioning satellite i to be used in positioning. A first row is a partial differential coefficient αi in the x direction of a pseudo distance ρi that can be calculated from a signal of the positioning satellite i. Second and third rows are partial differential coefficients βi and γi in the y and z directions of the same.
In FIG. 11, the design matrix has four lines, which means that a positioning calculation is performed using four satellites. In the positioning calculation, the number of positioning satellites is not limited to four.
The GDOP is defined by a square root of a diagonal element sum of (AT·A)−l. Here, AT is a transposed matrix of the matrix A, AT·A is a product of the transposed matrix AT and the matrix A, and (AT·A)−1 is an inverse matrix of the matrix (AT·A).
While the precision of a positioning calculation has been conventionally grasped using such indexes, degradation of the precision that cannot be determined by the DOP occurs due to quality degradation of a signal received from the each of the positioning satellites, degradation of trajectory information of each of the positioning satellites or the like.
The quality degradation of a signal is exemplified by the case in which a signal is not directly received from the positioning satellite but is reflected by an obstacle around the positioning satellite to be received, and the case in which, if an angle of elevation of the positioning satellite in a positioning position is low, a propagation distance in the atmosphere becomes longer to make a propagation delay larger.